What does focal length mean in Blender?
Focal length is the distance between the lens and the focal point:



At focal point the image is sharp and it's the place where the camera sensor is located (or camera film). The Field of View of each lens depends on the sensor size:



The conversion formula is this:

FOV = 2 * arctan(half_the_diagonal_of_sensor / focal_length)


Blender allows you to enter the "zoom" of camera in either Focal length (mm) or in Field of View (°):